Any light striking particles suspended in a liquid is scattered. The intensity of this light scattering is used in the visual turbidity measurement as a direct measure of the turbidity determination. In general, the light is guided through optical windows along its path from the light source to the medium to be measured as well as from the medium to the photodetector after light scattering, for example at a particle.
The measured scattered light intensity can be substantially negatively affected by the following two interference variables:
First, by reflections at walls and objects: The measuring signal is falsified if the light emitted from the sensor is reflected at walls or other objects or scattered at walls or other objects, and if these reflection and/or scattered signals that are not caused by suspended particles are detected by the sensor. This occurs, for example, in restricted installation spaces such as the measurement in pipes or fittings, or at very low turbidity, i.e. at a low extinction.
Second, due to any dirt on the optical window: In the case of window contamination such as deposits, biofilms, or adhering air bubbles, it is possible that light is scattered on the dirt and falls directly on the photodetector, without the light being scattered by suspended particles. Nevertheless, the light absorption caused by window contamination may also decrease the scattered light intensity that is received at the photodetector. Both cases lead to a falsification of the measured value.
Detection and/or compensation of the interference variable has been implemented until now either by model-based diagnosis method (for this, refer to German Patent, DE 10 2009 001 929 A1) or by multi-beam alternating light. Both methods require a plurality of light sources and/or photodetectors. However, this is not possible for many applications or does not comply with regulations, e.g. DIN ISO 7027 or EPA 180.1, which specify a single light source, a single photodetector and light intensity measurement at 90° for single light beam.